Suppressed ion chromatography (SIC) is a commonly practiced method of ion chromatography which generally uses two ion-exchange columns in series followed by a flow through conductivity detector for detecting sample ions. The first column, called the analytical or separation column, separates the analyte ions in a sample by elution of the analyte ions through the column. The analyte ions are flowed through the analytical column via a mobile phase comprising electrolyte. Generally, a dilute acid or base in deionized water is used as the mobile phase. From the analytical column, the separated analyte ions and mobile phase are then flowed to the second column, which is called the suppressor or stripper. The suppressor serves two primary purposes: (1) it lowers the background conductance of the mobile phase by retaining (e.g., suppressing) the electrolyte of the mobile phase, and (2) it enhances the conductance of the analyte ions by converting the analyte ions to their relatively more conductive acid (in anion analysis) or base (in cation analysis). The combination of these two functions enhances the signal to noise ratio, and, thus, improves the detection of the analyte ions in the detector. Accordingly, upon exiting the suppressor, the analyte ions and suppressed mobile phase are then flowed to the detector for detection of the analyte ions. A variety of different types of suppressor devices and methods are discussed in U.S. Pat. Nos. 3,897,213; 3,920,397; 3,925,019; 3,926,559; and U.S. Ser. No. 08/911,847. Applicants hereby incorporate by reference the entire disclosure of these patent applications and patents.
As those skilled in the art will appreciate, both the mobile phase and the sample contain counterions of the analyte ions. A suppressor operates by ion exchange of suppressor ions, which are located in the suppressor, with both the (1) the mobile phase electrolyte counterions and (2) the sample counterions. In anion analysis, for example, the suppressor ions normally comprise hydronium ions and the mobile phase comprises electrolyte such as sodium hydroxide or mixtures of sodium carbonate and sodium bicarbonate. In cation analysis, the suppressor ions normally comprise hydroxide ions, and the mobile phase may comprise electrolytes such as hydrochloric acid or methanesulfonic acid. The suppressor ions are located on a stationary phase, which may be an ion exchange membrane or resin. As the mobile phase and sample (which contains both analyte ions and counterions of the analyte ions) are flowed through the stationary phase of the suppressor, the electrolyte counterions in the mobile phase and the sample counterions are retained on the stationary phase by ion exchange with the suppressor ions. When the suppressor ions are either hydronium or hydroxide, ion exchange of the electrolyte counterions with suppressor ions converts the mobile phase to water or carbonic acid, which are relatively non-conductive. On the other hand, the ion exchange of sample counterions with suppressor ions (i.e., hydronium or hydroxide ions) converts the analyte ions to their relatively more conductive acid (in anion analysis) or base (in cation analysis). Thus, the analyte ions, which are now in their relatively more conductive acid or base form, are more sensitive to detection against the less conductive background of the mobile phase.
However, unless the suppressor ions are continuously replenished during the suppression process, the concentration of suppressor ions on the stationary phase is reduced. Eventually the suppressor will become exhausted and its suppression capacity is either lost completely or significantly reduced. Thus, the suppressor must be either replaced or regenerated. The need to replace or regenerate the suppressor is inconvenient, may require an interruption in sample analysis, or require complex valving or regeneration techniques known in the art. One example of a known technique for regenerating a suppressor by continuously replenishing suppressor ions is disclosed in U.S. Pat. No. 5,352,360.
In addition to the need for regenerating or replacing suppressor ions, another problem associated with SIC is that a separate suppressor unit is usually required, and, therefore, the number of components in the system is increased over traditional IC systems. Traditional IC systems usually contain a mobile phase source, a pump, a sample injector, an analytical column and a detector for detecting the sample ions. In SIC, a separate suppressor unit is added to the system. This, in turn, increases the complexity of the system and also increases extra-column volume which may decrease chromatographic resolution and sensitivity. Therefore, it would also be advantageous to have a system of ion suppression chromatography which reduced the number of system components in traditional SIC systems.
Another problem associated with prior art SIC systems is that the mobile phase is converted to a weakly ionized form, which renders the mobile phase unsuitable for reuse. Thus, it would be advantageous if a system of SIC were developed in which the mobile phase is converted back to its strongly ionized form after suppression and, thus, may be reused.